Most modern vehicles have power steering in which the force exerted by the operator on the steering wheel is assisted by hydraulic pressure from an electric or engine-driven pump. The force applied to the steering wheel is multiplied by the mechanical advantage of a steering gear. In many vehicles, the steering gear is a rack and pinion, while in others it is a recirculating ball type.
When operating at low speeds, hydraulic assist provides satisfactory feel and response characteristics and accommodates the excess capacity required for high-speed operation by the constant circulation of hydraulic fluid through a directional bypass valve. This bypass flow combined with system backpressure expends power needlessly from the vehicle powerplant. These losses are also a function of the rotational speed of the pump. Thus, hydraulic efficiency decreases with engine speed. Average losses under a no steering, zero speed condition can exceed 100 Watts.
Electric power steering is commonly used in the hybrid vehicles to improve fuel economy and has started to replace hydraulic power steering in some vehicles. One way this is accomplished is through the reduction or elimination of losses inherent in traditional steering systems. Therefore, electric power steering typically requires power only on demand. Commonly, in such systems an electronic controller is configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings. Electric power steering has several additional advantages. The steering feel provided to the operator has greater flexibility and adaptability. Overall system mass savings may also be achieved. Electric power steering is powerplant independent, which means it can operate during an all electric mode on a vehicle.
Furthermore, polyphase permanent magnet (PM) brushless motors excited with a sinusoidal field provide lower torque ripple, noise, and vibration when compared with those excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
One method of torque control for a permanent magnet motor with a sinusoidal, or trapezoidal back EMF is accomplished by controlling the motor phase currents so that the current vector is phase aligned with the back EMF. This control method is known as current mode control. In this a method, the motor torque is proportional to the magnitude of the current. However, current mode control requires a more complex controller for digital implementation and processing. The controller would also require multiple current sensors and A/D channels to digitize the feedback from current sensors, which would be placed on the motor phases for phase current measurements.
Another method of torque control is termed voltage mode control. In voltage mode control, the motor phase voltages are controlled in such a manner as to maintain the motor flux sinusoidal and motor backemf rather than current feedback is employed. Voltage mode control also typically provides for increased precision in control of the motor, while minimizing torque ripple. One application for an electric machine using voltage mode control is the electric power steering system (EPS) because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering. However, commercialization of EPS systems has been limited due to cost and performance challenges. Among the most challenging technical issues are a pulsation at the steering wheel and the audible noise associated with voltage mode control.
To satisfy the needs of certain applications for precise and accurate control, current mode control employing motor phase current feedback may be utilized, in lieu of the voltage mode control method wherein no current feedback occurs. That is, current mode control is a closed loop methodology in that a current feedback in addition to the motor position and speed feedback is used as controlling parameter by the current mode control, while voltage mode control is an open loop control methodology wherein only position and speed feedback is used. Voltage mode control systems may be more desirable in certain applications because the need for external sensors to provide feedback is minimized. However, it is nonetheless desirable to monitor and observe the characteristics of the motor to ensure proper function throughout the entire operational regime especially under dynamic operating conditions or to address changes in the motor parameters. For example, under temperature variations, the motor parameters and characteristics vary, often significantly.
EPS control systems employing voltage mode control algorithms, generally do not use the motor phase current for torque control. However, the current may still be used in a variety of algorithms such as motor parameter tuning, operation monitoring and diagnostics. Furthermore, in control algorithms with phase advancing, the phase current vector becomes a complex function of motor torque, speed, phase angle, and motor parameters. While the phase current is readily available for measurement, such measurement would require additional sensors and interfaces. Therefore, in a voltage control system, it is desirable to determine the motor phase current without relying upon current sensors or measurements.
A method and system for estimating current in a PM electric machine is disclosed. The method includes acquiring a torque value representative of the torque produced by the electric machine; receiving a position value indicative of the rotational position of the electric machine; obtaining a speed value indicative of the rotational velocity of the electric machine; receiving a temperature value representative of a temperature of the electric machine; calculating an estimate of the current. Where the calculating is based upon at least one of said torque value, said position value, said speed value, and said temperature value.
The system for estimating current in an electric machine comprises:
a PM electric machine; a position sensor configured to measure a rotor position of the electric machine and transmit a rotor position signal; a temperature sensor configured to measure a temperature of the electric machine transmit a temperature signal; a controller, which controller receives the rotor position signal and calculate a motor speed value; receives a temperature signal and generate a temperature value; calculates the estimate of the current.